SERDES - 4 ns random window

[alyxazon]

Originally posted by @dietrichb in #309 (comment) =>

There is another issue. If a 'fallout' node locks to White Rabbit, it might look at different 'positions' within a 4 ns window. Once locked, it will always remain locked at its initial position.

This issue becomes obvious if one compares two timing receivers (time stamping or digital output) with the same signal. The time difference will remain identical as long as none of the two timing receivers is restarted. But after a restart, the time difference might have a different value. This issue seems to be present for all form factors.

[alyxazon]

ToDo: Check LVDS PLL reset mechanism (locked? ref_rstn?)